This invention relates to an apparatus for end-rounding bristles for brushes and more particularly to an apparatus which can end-round bristles having a variety of heights.
Most brushware, especially toothbrushes, is produced via a two-part technique. The handles are produced by injection molding, following which the bristles are inserted into the handle. The most common technique for inserting the bristles into the brushes is stapling. The bristles are folded around a metal staple which is pushed into a pre-molded hole in the brush. The staple cuts into the plastic at the periphery of the hole, and the plastic retains both it and the bristles.
In an alternative technique, the bristles are fused with the handle. Either the bristles or the brushhead, or both, are heated, and the bristles are inserted into holes in the handle where they are retained by the cooling plastic. Exemplary techniques for brush production by fusion include those described in U.S. Pat. No. 4,988,146, which describes a fusion process wherein the ends of bristle bundles are thermally fused, shortening and locally thickening the bundles to form a fuse-ball, or fuse, which is inserted into a hole in a brush handle. However, it is not necessary to use a handle with prefabricated holes; the holes can be formed immediately prior to the insertion of the bristles. In a process called xe2x80x9chedgehogging,xe2x80x9d the handle is heated, and a set of short spikes mounted on a heated plate is pushed into the head of the handle to form holes to receive the bristles. U.S. Pat. No. 4,637,660 describes an exemplary hedgehogging process wherein, as the holes are formed in the handle, the displaced material is organized into a small bead surrounding the newly-formed hole. As in the ""146 patent, the bristle ends are fused before they are inserted into the hedgehogged holes. Material from the small bead flows around the fuse after it is fitted into the hole, enclosing the bristles in the brush head.
In an alternative technique, called xe2x80x9cin-mold bristling,xe2x80x9d the handle is molded around the bristles. As in the fusion processes described above, the ends of the bristles are fused. The fused ends of the bristles are held in a mold cavity into which the material for the handle is injected. The cooled material becomes the head of the brush and also retains the bristles. An exemplary in-mold technique is disclosed in U.S. Pat. No. 5,143,424.
In each of these techniques, the use-ends of the bristles must be polished, or end-rounded, to remove rough edges which result from trimming. The sharp edges can cut into the gums of a consumer, causing pain and bleeding. The bristles are fed into the brushmaking apparatus from a pre-cut supply or trimmed from an endless supply, or creel, as each brush is produced. The bristles may be inserted directly into the brush, as for a staple-set process, or into a magazine in which the bristles are further processed before being united with the handle.
U.S. Pat. Nos. 5,431,484 and 5,518,300 disclose methods for end-rounding bristles before they are inserted into a brush. The ""484 patent teaches that the free end of an endless supply of bristles is guided to a grinding device and supported at a predetermined distance from the bristle ends. The pre-determined distance is selected to control the flexure of the bristles as they are polished by a rotating plate on the grinding device. After the bristle ends are end-rounded, they are cut to the proper length for insertion into a brush. The ""300 patent teaches a method of producing a staple-set brush in which the bristles are polished after being removed from a pre-cut supply but before insertion into the brush.
U.S. Pat. No. 4,979,782 discloses a brush production method in which the bristles are end-rounded before insertion into the handle. The bristles are clamped so that their ends are in a flat plane. The bristles are then end-rounded, following which they may be profiled to take on a variety of lengths. After profiling, the bristles are inserted into a brush.
U.S. Pat. No. 5,176,427 discloses a method of varying the flexure of bristles for end-rounding. A movable clamp is adjusted to retain the bristles at a fixed distance from their use ends. The distance can be varied but is the same for all bristles, as they are polished while their use ends are in a plane.
U.S. Pat. Nos. 5,649,851 and 5,653,628 both describe end-rounding bristles with a rotating cylinder. In the ""851 patent, a brush, for example, a toothbrush, is held against an abrasive strip on the outside of a rotating cylinder. As the cylinder rotates, the abrasive strip polishes the ends of the bristles. The ""628 patent discloses an apparatus and method for polishing the bristles of a cylindrical brush. In contrast to the ""851 patent, the abrasive is disposed on the inside of the cylinder rather than the outside. The cylindrical brush is passed into the cylinder and either the cylinder or the brush rotated to pass the bristles over the abrasive, end-rounding the bristles.
Modem brushes generally contain bristles having a variety of lengths. Both the individual bristles in a bundle or tuft and the tufts themselves may vary widely in length. Prior art end-rounding methods employ flat, diamond-coated plates which vibrate in an eccentric, two-dimensional path as the bristles are held against them. This method works well for brushes having bristles of one length but is less effective for bristles of varying length. As the difference in bristle height increases, the force on the longer bristles increases, and the finish quality of the shorter bristles decreases. Bristle bundles with significantly different heights are typically not polished in a single step. Instead, the longer bristles are pushed aside by a sleeve while the shorter bristles are polished, following which they are released and end-rounded. However, if the longer and shorter bristle bundles are interspersed, the sleeve cannot mask the longer bristles without also masking the shorter ones.
In one aspect, the invention is an apparatus for end-rounding bristles for a brush. The apparatus includes a plurality of pins disposed on a base and an abrasive coating disposed on at least an end of the pins. When a plurality of bristles are disposed adjacent to the pins, two-dimensional motion of the base will cause the pins to polish ends of the bristles. The pins may be configured to polish bristles having a plurality of lengths simultaneously, and the apparatus may further comprise retaining means to hold the plurality of bristles adjacent to the pins. The pins may be adapted and constructed to have a length such that an overlap between an end of the pin and an end of a bristle contacted by the pin is between 0.005 and 0.050 inches when the plurality of bristles is adjacent to the pins. The pins may be arranged on the base such that, when the plate is in motion, each pin polishes ends of a pre-determined bundle of bristles. The bristle bundles may vary in height such that they could not all be polished simultaneously with planar polishing means, and the ends of the bristles need not be oriented perpendicular to a plane of motion of the base. The pins may be constructed and arranged to only polish a portion of the plurality of bristles. An end of each of the pins may comprise a partially rounded tip having a radius of curvature between 0.075 and 0.08 inches. The tip may be truncated in a flat plane having a truncation depth between 2.5 and 65% of the radius of curvature. The diameter of each pin may be at least xe2x85x9 inches, at least {fraction (5/32)} inches, or at least as long as a lateral dimension of a pattern defined by the plurality of bristles. The abrasive coating may comprise a diamond coating having a grit size between 200 and 600 grit.
In another aspect, the invention is a method of end-rounding bristles in a plurality of bundles. The method comprises selecting a two-dimensional motion path for a plate, identifying a point on the plate that travels a path intercepting a position of a first bundle of bristles that is disposed in a polishing position with respect to the plate, and selecting an abrasive pin to place at the identified point. The pin has a height such that, when the first bristle bundle is disposed in the polishing position, the pin will polish ends of bristles in the first bristle bundle. The steps of identifying and selecting a pin may be repeated for a subsequent point corresponding to a second bundle of bristles, and the average height of the bristles in the first and second bundles of bristles need not be the same. The pins may be constructed and arranged to polish a portion of a plurality of bristle bundles that vary in height such that all the bristle bundles could not be polished simultaneously with planar polishing means, and the ends of the bristles need not be oriented perpendicular to a plane of motion of the plate.
In another aspect, the invention is a method of end-rounding a plurality of bristles. The method comprises disposing ends of the bristles in a path of a translatable abrasive pin and moving the pin such that it polishes ends of the bristles. The bristles may be disposed in at least one bundle of a plurality of bundles having a variety of heights such that all the bristle bundles could not be polished simultaneously with planar polishing means. The path of the pin may comprise a four-lobed pattern that defines two perpendicular axes. Alternatively, the path of the pin may comprise two superimposed four-lobed patterns, wherein each set of four lobes defines two perpendicular axes. The method may be adapted for use with a plurality of pins or a plurality of bristle bundles, or both.